This invention relates to the field of geophysical prospecting and, more particularly, to a method to calculate and apply a time correction to correct for propagation through the variable velocity in the water layer.
In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. Generally, a seismic energy source is used to generate a seismic signal which propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
One of the problems arising in acquiring and processing 3D seismic data in the some marine areas is that of variable water velocity creating inconsistent traveltimes between sources and receivers. As a result of the interaction between warm and cold currents, the water velocity may vary relatively rapidly, both temporally and spatially. In such areas, these velocity variations may be large enough to have a detrimental effect on subsequent data processing.
Water velocity can be related to the water temperature, salinity and depth. The velocity of compressional waves in water has been modeled as [Sheriff, Encyclopedic Dictionary of Exploration Geophysics, 3rd Ed., 1991]:
V=1449.2+4.6Txe2x88x920.055T2+0.0003T3+(1.34xe2x88x920.010T)(Sxe2x88x9235)+0.0162Z 
where V is the velocity in meters per second, T the temperature in degrees Celsius, S the salinity in parts per thousand and the depth below the water surface, Z, is in meters. As bodies of water having differing temperatures meet, there can be different degrees of mixing and the location of the xe2x80x98frontxe2x80x99 between the bodies of water can move significantly over short periods of time. This means that the structure of the water column and hence the water velocity structure can change significantly over relatively short distances and times.
These water velocity changes have implications for seismic processing. For a typical 3D acquisition scenario, the water velocity will vary along the length of a sail-line. In some situations it is possible to identify and track reflections from interfaces between the warm and cold layers of water along the length of a sail-line. Water velocity changes over distances of less length of the acquisition cable may effect the acquisition. However, the primary effect is usually between adjacent sail-lines. Depending upon the acquisition timetable, adjacent or areally coincident sail-lines may be shot hours, days or even weeks apart, so although two lines may be physically close to one another they can be widely separated in time, possibly resulting in datasets with significant water velocity differences. Water velocity differences will result in dynamic differences between data in the combined datasets and these change may effect the data processing, in particular processes like multiple attenuation, DMO, stacking and 3D migration.
Without an independent measurement, the prior art methods have usually estimated the water velocity from the seismic data itself. Direct arrivals are not useful since they travel only through shallow water and often do not have sufficient information to determine the actual depth to velocity profile. Water velocity will generally increase with depth. As water depths increase the effects of water currents become stronger. If there is no direct measurement of the water depth, the water bottom time cannot be used to compute the water velocity. If there are no significant velocity changes within the acquisition spread length, a stacking or RMS velocity may be estimated from shot records or CMP gathers. Also, tomographic or wave-equation based inversion techniques might be used to invert for a depth (and perhaps, spatially) varying velocity function.
One prior art approach for water velocity determination has been to derive an estimate from the stacking velocity. This method assumes that the water velocity is constant, or varies only with depth, giving approximately hyperbolic moveout and lateral variations in the velocity may be ignored. This calculation is restricted to near offsets so as to be unaffected by refracted events. A local average water velocity may be estimated in this manner.
Changes in the water velocity resulting in dynamic differences in the combined seismic data may often be large enough to effect data processing and subsequent imaging. For 2D processing or within each sail-line the effect may not be particularly dramatic since the velocity changes can be relatively smooth and continuous along the acquisition spread. However for 3D processing, where temporally different but areally coincident or adjacent data are combined, the water velocity may change discontinuously. The data may appear to contain discontinuities or xe2x80x98bustsxe2x80x99 in, for example, the crossline direction of adjacent sail-lines.
Detrimental effects upon subsequent processing due to apparent static problems will occur if these problems are not remedied. For example, 3D DMO will not function adequately and smear artifacts will be problematic. The 3D migration will produce artifacts at all azimuths from the edge of each bust. Many 3D pre-stack data processes may be compromised.
Water velocity changes on seismic data records are dynamic and the cost of applying a proper correction, for example through a wave-equation redatuming approach, may be computationally expensive. One prior art method has been to apply a static correction. For a flat water bottom and a depth independent water velocity, the change in the water bottom arrival time, xcex94tw, due to a change in water velocity, xcex94Vw, is straightforward to calculate:
xcex94tw=tw(xcex94Vw/Vw) 
This calculation can be used to compute a static correction to compensate for a change in the water velocity by computing a static for the zero-offset case and applying it after normal moveout has been applied to the data. As the correction is accurate only for the zero-offset case, slanted ray paths will not be accounted for accurately.
The water velocity and water bottom zero-offset time may be computed along the length of each sail-line and then used to generate the static correction described above, correcting the data assuming a change in the water velocity from a reference velocity. These corrections vary continuously along each sail-line, but are discontinuous across sail-line boundaries or between areally coincident datasets acquired separately in time. The static corrections may then be applied to the data after applying NMO.
Oceanographic conditions can effect the water velocity over relatively short time spans and/or small spatial distances. Because areally adjacent or coincident data may be acquired at different times, giving time for the water velocity to change, discontinuities may occur which can seriously effect later processing. Prior art methods only partially compensate for temporally changing water velocities that affect acquisition. It would be advantageous to have an efficient correction for dynamically changing water velocities in marine data acquisition that accounts for slanted ray paths.
The invention is a method of deriving a velocity correction for seismic data processing to correct for the effects of variable water velocities. A zero-offset static correction is determined for the seismic data that is the difference between an observed time to a water bottom and an ideal time to a water bottom determined using a selected ideal velocity. An ideal water velocity is selected for the seismic data. A zero-offset water bottom time is determined for the seismic data. An observed velocity is determined for the seismic data. A dynamic water velocity correction is computed and applied to the seismic data for varying offsets and raypaths.